1. Field of the Invention
The present invention relates principally to a noise eliminator for a radio receiver, and more particularly to an improvement capable of achieving effective elimination of an impulse noise such as an ignition noise from an internal combustion engine.
2. Description of the Prior Art
In advance of an explanation on the noise eliminator of the present invention, a conventional device of this kind will be described briefly.
FIG. 1 shows a block diagram of a known noise eliminator which comprises a noise detection circuit 3 for detecting an impulse noise signal which arrives from an FM detector 2 in an FM receiver, a control-pulse forming circuit 4 for producing a control pulse signal out of the noise signal detected by the circuit 3, and gate circuits 5 for controlling the signal paths at outputs of a multiplex demodulator 12 by the control pulse signal obtained from the circuit 4. In this network, an FM noise is detected by introducing the FM detection output into a 100 KHz high-pass filter 6 and a rectifier 7. The noise signal thus detected is amplified by an amplifier 8 and then is fed to a monostable multivibrator 9 in the control-pulse forming circuit 4 so as to trigger the multivibrator 9. After being integrated by a unidirectional integrator 10, the output of the multivibrator 9 is shaped into a desired waveform by a waveform shaper, e.g. a Schmitt trigger circuit 11 to become a control pulse signal, which is then fed to the gate circuits 5. In response to arrival of the control pulse signal, the gate circuits 5 serve to interrupt the output signal paths of the multiplex stereo demodulator 12 to prevent the noise from appearing in the demodulated output signals.
In such a noise eliminator as described above, however, there exists difficulty in properly setting the absolute triggering level for the monostable circuit 9 due to the variety of the noise component level which varies in accordance with the antenna input level. If the triggering level is relatively low, the gate circuits are frequently rendered nonconductive and the transmission of the demodulator output signals is unnecessarily interrupted resulting in poor efficiency of the signal transmission. On the contrary, when the triggering level is relatively high, the impulse noise will not be eliminated effectively.